This invention relates to an electro-optical conversion device for mutually changing over the bi-stable state of a ferroelectric liquid crystal and driving the same. More particularly, the present invention contemplates to drive most suitably the electro-optical conversion device described above. The electro-optical device according to the invention may be utilized as a display device, an optical shutter for a printer or the else.
There has been known in the past a ferroelectric liquid crystal electro-optical device of the driving system which changes over the bi-stable state of a ferroelectric liquid crystal by a pulse having a peak value above a threshold voltage to drive the liquid crystal and holds the bi-stable state after switching by an A.C. pulse. Such a device is described, for example, in the article of SID'85 Int'l Symposium 16, 131(1985).
First of all, the structure of a conventional ferroelectric liquid crystal cell (hereinafter called the "liquid crystal cell") is shown in FIG. 2. Reference numeral 1-1 represents a pair of substrates that are arranged to face each other. Reference numeral 3 represents a thin film of a ferroelectric liquid crystal such as a chiral smectic C liquid crystal (hereinafter called "SmC*") sandwiched between the substrates 1-1.
Reference numeral 2-2 represents uniaxial and random horizontal orientation films that exist on the interfaces between the substrates 1-1 and the SmC* thin films and accomplish the bi-stable state of liquid crystal molecules. The major axes of the liquid crystal molecules (hereinafter called the "molecular axes") extend horizontally with respect of the substrate 1 and form a layer. When observed from the top, the liquid crystal molecules are divided into two domains. In the first domain, the molecular axes are inclined by +.theta. relative to the normal 4 of the layer. This is the first stable state 5. Spontaneous polarization 7 of the liquid crystal molecules faces upward. The second domain is inclined by -.theta. relative to the normal 4 of the layer. This is the second stable state 6.
At this time spontaneous polarization 7 faces downward. Either one of these bi-stable state is selected by positive and negative A.C. pulses by utilizing the property of spontaneous polarization 7 that its direction is opposite under the bistable state. Reference numeral 8-8 represents a pair of polarizers arranged with their axes of polarization crossing each other perpendicularly. They distinguish optically the bi-stable state by birefringence. For example, they convert the first stable state to a light cut-off state (hereinafter called "black") and the second stable state to a light transmission state (hereinafter called "white"). Reference numerals 9 and 10 represent matrix electrodes for applying driving voltages to the SmC* thin film 3. As shown in FIG. 3, reference numeral 9 represents scanning electrodes (hereinafter called "strobe") and 10 does signal electrodes (hereinafter called "signal").
FIG. 4 shows a driving waveform applied to one matrix pixel (hereinafter called "dot") in line-sequence driving by use of an A.C. bias averaging method. Positive and negative (with reference to the strobe 9) pulses P.sub.1 and P.sub.2 having peak values above a threshold voltage are applied continuously during the selection period in a first frame. The liquid crystal molecules are aligned to the second stable state by the positive pulse P.sub.1 and switched and aligned to the first stable state by the subsequent negative pulse P.sub.2. This period is called the "selection period". This state is held by the application of A.C. pulses consisting of subsequent pulses P.sub.3 and P.sub.4 because the peak values of the A.C. pulses are below the threshold value. This state is called the "half-selection state". Therefore, black as the first stable state is written in the first frame. In the subsequent second frame, white is written because the polarity of the pulse is opposite. However, in the drawing, white is not written because P.sub.5 and P.sub.6 are below the threshold value, and black that has been written in the first frame is held as such. The period of P.sub.5 and P.sub.6 is called the "non-selection period". FIG. 4B shows the result of measurement of the change of the transmission light intensity at this time measured by a photomultiplier.
Here, the peak values of the pulses of the selection period P.sub.1 and P.sub.2, the half-selection period P.sub.3 and P.sub.4 and the non-selection period P.sub.5 and P.sub.6 are selected so as to satisfy the following relationship with V representing the absolute value of the pulses P.sub.1 and P.sub.2 : EQU .vertline.P.sub.3 .vertline.=.vertline.P.sub.4 .vertline.=V/N, EQU .vertline.P.sub.5 .vertline.=.vertline.P.sub.6 .vertline.=V.(B-2)/B
where B is a bias value.
When driving time-divisionally a heretofore known twisted nematic liquid crystal, there is known a voltage averaging method proposed by Alt and Pleshko (IEEE Trans. Ed, 1974, ED21, pp 146-155). They proposed also the optimum driving condition in this method.
However, this method cannot be applied to SmC* for the following reason. Namely, though the change of transmission light intensity of the twisted nematic liquid crystal depends on the effective voltage value, the SmC* liquid crystal depends on the absolute value of the voltage. Therefore, the driving method as well as the circuit are different between them and the driving conditions change naturally, too.
No report has been made to this date on the optimum driving condition when SmC* is driven time-divisionally, and it has been difficult to represent the optimum condition when driving SmC practically.